In recent years, magnetic disk apparatuses such as a hard disk, are required not only to be improved in recording density and recording capacity, but also to be smaller and thinner.
A commonly-used hard disk includes, as shown in FIG. 4, a magnetic disk medium 1, a spindle motor 2, a magnetic head assembly 3, a voice coil motor 4, and a circuit component 5.
According to a cross-sectional view at the surface including the rotation center of the magnetic disk medium 1 shown in FIG. 5, the spindle motor 2 includes a motor hub 21, permanent magnets 22 arranged along the circumference at the outer periphery side in the motor hub, motor coils 24 formed by wires wound therearound on a stator base 23, and a magnetic shield 25 for preventing a magnetic field generated in the spindle motor 2 from affecting magnetic signals recorded in the magnetic disk medium 1.
The motor hub 21 supports the magnetic disk medium 1. When rotating the spindle motor 2, the motor hub 21 including the permanent magnets 22 therein, is excited by using respective motor coils 24 and rotationally driven. In the shown example, the spindle motor 2 is driven by a three phase alternating current, therefore, multiples of “3” (in this case, 9) motor coils 24 are provided.
In the spindle motor 2, the motor coils are arranged on the circumference at almost equal intervals as further shown by a plan view thereof in FIG. 6.
On the other hand, the magnetic head assembly 3 pivotally moves about the rotation center at an angle of a prescribed range by the voice coil motor 4. At this time, when moving the head to a track of the most-inner circumference in the magnetic disk medium 1, the head will be moved to a gap between the magnetic disk medium 1 and the stator base 23 (gap denoted by “D” in FIG. 5).
The circuit component 5 is connected to a host such as a personal computer. The circuit component 5 rotationally drives the spindle motor 2, and rotationally drives the voice coil motor 4 so as to move the head to a track which is an access target in tracks formed on the magnetic disk medium 1 in accordance with an instruction of access inputted from the host side. The circuit component 5 also outputs data to the host, which is demodulated from a signal read out from the magnetic disk medium 1 by the head installed in the magnetic head assembly 3. Further, when receiving input of data to be recorded from the host, the circuit component 5 outputs a signal relating to the data to the head installed in the magnetic head assembly 3, and records the data on the magnetic disk medium 1.
As described above in reference to FIGS. 4 and 5, the head for reading or writing information is transported on the magnetic disk medium 1, therefore, it is necessary to install a member (motor coil assembly) including the motor coil assembly stator base 23 at the position apart from the surface of the magnetic disk medium 1, in consideration of the thickness of the head.
Therefore, even when the apparatus is intended to be thinner, it is difficult to narrow the gap (“D” in FIG. 5). When the gap is narrowed in order to make the apparatus thinner, a portion relating to the circumference of the stator base 23 is not used in the inner circumference side of the magnetic disk medium 1, or the mounting volume of the motor coils 24 is made to be small.
However, when the inner circumference side in the magnetic disk medium 1 is not used, recording capacity will be reduced. When the mounting volume of the motor coils 24 becomes small, driving ability will be reduced and starting time of the spindle motor will be long.
In Japanese Laid-Open Patent No. 11-110896 (“Patent Document 1”), a technique is disclosed in which the stator coil has a shape of axis asymmetry having a notch which is larger than a range interfering with a moving range of the magnetic head to prevent the interference between the magnetic head and the stator coil.